Planographic printing plates



United States Patent 3,020,839 PLANOGRAPHIC PRINTING PLATES Leslie E. Richard, South Hadley Center, Mass., assignor to S. D. Warren Company, Boston, Mass., a corporation of Massachusetts No Drawing. Filed Sept. 21, 1959, Ser. No. 840,999 5 Claims. (Cl. 101-1491) This invention relates to coated paper base planographic printing plates or masters and to a method of making such plates.

Planographic printing is printing from plane surfaces. that is level surfaces without either raised or intaglio areas. Such printing depends upon the immiscibility of grease and water, or more specifically, of greasy lithographic printing-ink and aqueous lithographic solutions. To print by the planographic printing process or plane lithography there is afiixed to a suitable printing surface or plate a water-repellent image usually greasy, resinous, or waxy in nature. To the plate so imaged is then applied aqueous wet-out liquid, such as a solution of glycerine slightly acidified with phosphoric acid, acid phosphates or similar substances. The aqueous wet-out liquid wets all portions of the plate surface not already covered by the water-repellent imaging material, but does not, of course, wet the water-repellent image. An inking-roll coated with lithographic printing ink now passed across the surface of the plate leaves a film of ink upon the inkreceptive imaged areas but leaves no ink on the unimaged areas which are already covered by a film of ink-repellent aqueous liquid. When the inked plate is brought into contact with another ink-receptive surface it transfers ink thereto in a pattern reverse to that of the plate. The surface so printed upon may be a paper sheet, but in most cases is a rubber offset blanket which in turn transfers the print in original form to a paper sheet which is the final printed matter. The plate is repeatedly dampened, inked, and printed from until the desired number of prints have been made.

A requisite for a satisfactory plate is that when dry the plate shall readily accept a water-repellent image which must adhere tenaciously thereto and shall neither be pulled away by printing ink nor be washed away by aqueous lithographic solutions. Another requisite is that all unimaged areas of the plate shall be readily wetted by aqueous lithographic solution and shall hold a film there of on the surface of said unimaged areas and shall not permit the aqueous film to be displaced therefrom by printing ink. These requisites are obtained by using paper planographic printing plates made by the process herein described.

Coated paper planographic printing plates or masters fall into two general classes. The first class comprises those which are photosensitized and then have a photographic image formed thereon. The other class comprises masters on which images are directly affixed by a writing machine such as a typewriter or by hand with a greasy crayon, ball-point pen, or the like. This latter class of masters, commonly designated as direct-image masters, is the class with which the present invention is primarily concerned.

In general a direct-image coated paper planographic master may be said to comprise a paper base, which preferably but not necessarily possesses wetstrength properties, said paper base bearing on at least one side a layer of coating containing finely divided mineral filler or pigment material such as clay, blanc fixe, titanium dioxide, or the like bound together and to the paper base by a considerably lesser weight of a water-insoluble hydrophilic film-forming adhesive material such as insolubilized casein, polyvinyl alcohol, starch, carboxymethyl cellulose,

3,020,839 Patented Feb. 13, 1962 hydroxyethyl cellulose, carboxymethyl starch, hydroxyethyl starch, polyacrylic acid, or the like. Since for each 100 parts by weight of pigment the weight of adhesive probably always falls between 10 and 75 parts, and usually between 15 and 50 parts, it is apparent that the dried layer of coating will be microscopically porous. The roughness or porous structure contributed by the pigment apparently is necessary to ensure adequate anchorage of images applied by typewriter ribbon, crayons, or the like. This same roughness, however, tends to make the unimaged background more liable to toning or contamination by ink during the printing operation.

It is well known that such toning can be prevented substantially entirely by incorporating in the surface pores of the master a substantial quantity of a water-soluble salt of a divalent metal, e.g. zinc acetate as disclosed in US. Patent No. 2,534,650. It appears that when a master containing such a divalent metal salt is wet with an aqueous lithographic wet-out or fountain-solution which contains the customary phosphoric acid or acid phosphate, then there is formed a jelly-like fairly insoluble reaction product of phosphate and divalent metal which is highlyetfective in holding aqueous lithographic solution on the surface of the master and thus preventing contamination of unimaged areas.

Properly made masters containing such soluble divalent metal salts run with remarkably clean unimaged areas. When eventual failure occurs, it is not because of toning but because the images gradually deteriorate or wash off from the master. The image is apparently undermined by water creeping between the image and the surface to which it is affixed. This possibly may be due to the gradual dissolving away of the divalent metal phosphate from beneath the imaged areas or perhaps to a physical change in the metal phosphate itself, or it may be due to the fact that the ingredients of the coating tend to adsorb water preferentially to imaging material or lithographic ink.

Failure of image by wash-off is not to be confused with failure by lifting out of the image along with fibers from the underlying base. Such lifting of fibers is practically inevitable from any paper-base master after long running unless the paper has been well impregnated with a water-resistant resin or coated with an impenetrable barrier coat. For this reason it is customary to provide so-called long-run coated paper planographic masters with a water-impenetrable barrier coat which isolates the paper base from the planographic printing coating. Such a barrier will not by itself, however, prevent wash-01f of the image referred to above, although it may delay such wash-off to some extent.

It is to be understood that planographic printing masters are intended primarily for use on rotary offset duplicators which are customarily operated by ofiice personnel rather than by skilled lithographers. The great majority of such masters are imaged by means of a typewriter or similar writing-machine. The ribbons used on such writing-machines fall into two general classes. One class is made up of fabric ribbons saturated with greasy ink, which can be used for a considerable time before they become worn out. The other class comprises single-use ribbons of paper or impenetrable film bearing thereon a waxy transfer coating. The latter ribbons usually give crisper and more attractive images than fabric ribbons.

Masters containing such soluble divalent metal salts in their writing surfaces are generally intended for use in the shortor medium-run category; that is, they are not expected to print more than a thousand copies. It is true that a skilled operator can usually obtain several thousand satisfactory prints from such a master. On the other hand an unusually inept operator might lose the image from such a master in somewhat less than one thousand prints.

Until such time as the image fails, however, such masters are practically foolproof in that unimaged areas tend to remain exceptionally clean and accidental smudges and fingerprints are easily removed without damage to either background or image.

In many cases it is desired to print more copies than can be reliably obtained from use of a medium-run master. Masters designed for such longer runs have been made having a structure more or less like that of the masters hereinbefore described, but having interstices and pores of the surface filled with a mixture of soluble acetates or formates of multivalent metals along with acetates or formates of alkali metals or ammonium. In this connection multivalent means having a valence of three or more. It is known that the presence of the alkali-metal or ammonium salt is effective in greatly slowing up the tendency of the multivalent metal acetate or formate to develop water-shedding and oleophilic qualities as they age (see US. Patent No. 2,635,537). Coated paper masters containing in the pores of their surface such multivalent metal acetate or formate buffered by sodium acetate or formate are in wide use and give excellent results in the hands of careful operators. In the hands of inept operators results may be less satisfactory. Such masters are definitely more sensitive than similar masters containing only divalent metal salts, and inadvertent smudges and finger-prints are more difiicult to remove therefrom. It appears that the dry multivalent metal salt itself is somewhate oleophilic, so that when once wet with imaging material or other greasy matter the latter is not easily displaced therefrom. This of course is advantageous from the standpoint of image life although it likewise makes the surfaces more sensitive and necessitates more careful handling in preparing the master for use and more care in maintaining proper fountain balance in printing from the master.

There are available on the market excellent coated paper masters containing acetates or formates of multivalent metals bufiiered with acetates or formates having monovalent cations. Some of these masters give excellent printing results when imaged by use of fabric typewriter ribbons. Others give excellent results when imaged by use of ribbons of the single-use type, such as paper base, acetate, or Mylar carbon ribbons. So far, however, no single master of this type has appeared which is wholly satisfactory for use with both fabric and single-use ribbons.

The present invention uses in the surface of a coated paper planographic printing master a mixture of specific divalent metal salts and specific multivalent metal salts. It makes possible the production of masters having a longer image-life than that obtainable with divalent metal salts alone. At the same time the masters are less sensitive than those containing multivalent metal salts buffered by salts with monovalent cations. Moreover by this invention masters can be produced which give excellent results when imaged by either a fabric ribbon or by a singleuse ribbon.

It is understood, of course, that in the practice of this invention the quantity of the soluble salts left in the surface pores of the master is in substantial excess of any amount which may be used up by reaction with the adhesive with which the salts may come into contact. That is, there must remain in the pores or interstices of the coating surface a substantial quantity of the soluble salts which are free to react with an ingredient of the aqueous lithographic solutions which are normally applied to planographic printing masters in use.

The optimum ratio of divalent to multivalent metal salts in any particular case will depend upon the particular qualities desired in the finished product as well as upon the particular salts used. In general it may be expected that a master containing in its surface pores about equal quantities by weight of divalent metal salt and multivalent metal salt will be practically as foolproof to use as one containing only the divalent metal salt. At the same time its maximum life in use may be expected to be considerably longer than that of the master containing only the divalent metal salt and in fact should be approximately the same as the life of a master containing only multivalent metal salt buffered by sodium acetate.

Of the divalent metal salts suitable for use, zinc acetate and zinc formate are greatly to be preferred, both because they are colorless and because their beneficial effect is far superior to that of other divalent metal salts. Multivalent metal salts which have been used successfully with zinc acetate and/or zinc formate include aluminum acetate, aluminum chloride, aluminum formate, aluminum sulfate, cerium chloride, ferric formate, and zirconium acetate. As little as one part by weight of any of these multivalent metal salts mixed with 9 parts by weight of zinc acetate and/or zinc formate and included in the surface pores and interstices of the coating of a coated paper planographic printing plate results in a plate which will print an appreciably greater number of acceptable copies than will be a similar plate containing an equal weight of zinc acetate only.

On the other hand, a small quanity of zinc acetate or zinc formate mixed with one of the multivalent salts mentioned above will when the mixture is included in the pores of a coated paper planographic printing plate result in a plate having an appreciably longer shelf life than a similar plate containing an equal weight of the multivalent metal salt alone. The minimum quantity of zinc acetate or zinc formate necessarily added to a multivalent metal salt to produce a planographic printing plate having any specified length of shelf life will vary considerably depending upon the specific multivalent metal salt used in the combination.

The mixture of zinc salt and multivalent metal salt in aqueous solution can be washed over the surface of the coated paper web to fill the pores and interstices of the coating, and if desired the solution may also contain a viscosity-imparting agent such as guar gum, locust bean gum, gum arabic, mesquite gum, hydroxyethyl cellulose, and the like. Alternatively, in cases where the adhesive is one, like starch, which is not coagulated by the metal salts, the salt mixture may be included in the coating composition itself before the paper web is coated therewith.

The teaching of the present invention may be used in making coated paper planographic printing plates intended for short or medium length runs of printing. The invention is particularly advantageous in preparation of coated paper planographic printing plates for printing comparatively long runs, as upwards of five thousand copies. The following Example 1 is an embodiment of the invention suitable for printing such long runs.

EXAMPLE 1 Step 1.A wet-strength paper web of approximately pounds weight, based on a ream of 500 sheets cut to a size of 25 x 38 inches, may be prepared from a papermaking furnish containing beaten fibers from deciduous trees and from coniferous trees in approximately equal quantities, rosin-size, melamine-formaldehyde resin, and alum. The fiber from deciduous trees may be replaced in whole or in part by fiber from coniferous trees, and clay or other suitable filler, although not particularly desirable, may be included in the furnish. The formed web is then preferably but not necessarily surface-sized with an aqueous solution of ammoniacal casein made slightly acidic with formaldehyde. Between one and two pounds (dry weight) of the casein size, per ream, should be taken up. The web may then be dried and machine-calendered.

In place of the materials employed to impart wetstrength to the paper base sheet made in accordance with the foregoing Step 1, namely, the melamine-formaldehyde resin and the ammoniacal casein-formaldehyde wash surface-size, other known materials may be employed for imparting wet-strength to the paper base sheet and among these are urea-formaldehyde resin, glue-formaldehyde,

viscose, and other materials. The use of the caseinformaldehyde wash, referred to in the foregoing Step 1, alone will provide a usable paper base sheet having fairly good wet-strength, but I prefer to use both it and the melamine-formaldehyde resin.

Step 2.-A paper base sheet web prepared in the manner set forth in the foregoing Step 1 may be coated on one side, which will be the reverse or non-printing side of the finished paper plate, with about pounds, dry weight, per ream of a composition containing 100 parts of clay, '18 parts of casein, and 1% parts of dimethylol urea, and the sheet is again dried. The function of this coat is merely to promote flatness of the finished product by counteracting the tendency to curl caused by subsequent application of coating to the other, or printing side, of the sheet. I

Step 3.To the previously uncoated side of the paper web may be applied about 10 pounds dry weight, per ream, of a barrier coat deposited from an aqueous composition containing dry weights according to the following Formula 1.

Parts by weight The coating is then well dried.

The term primary emulsion as used in preceding Formula 1 is used to designate an aqueous emulsion in which the polymeric substance therein has been polymerized in situ. The purpose of the barrier coat of Formula 1 is to prevent saturation of the paper fibers by fountain-solution during long runs; any barrier coat may be used which will accomplish this purpose and which is sufiiciently non-tacky so that the coated paper may be rolled up without sticking together and which is satisfactorily wettable by the aqueous coating composition of hydrophilic material to be subsequently applied thereto. Other film-formers which have been found useful for barrier coats include urea-formaldehyde resin, butadieneacrylonitrile copolymer, polymerized methyl acrylate, plasticized polyvinyl chloride, plasticized polyvinyl butyral, and the like. The inclusion of mineral filler or pigment in the barrier coat is optional; the filler acts to decrease tackiness and also probably serves as an anchor for the coating subsequently applied thereto. Step 4.-A coated paper web as prepared in the manner set forth in the foregoing Step 3 may be coated on top of the barrier coat of Step 3, with from 8 to 14 pounds, with an optimum of 12 pounds per ream, dry weight, of a mixture having the composition set forth in the following Formula 2 so as to provide a pigmented hydrophilic coating containing interstices which provide capillary spaces or pores:

Formula 2: Parts by weight Pigment (finely divided clay) 100.0 Hydrophilic adhesive (casein water solution of amonia) 18.0 Dimethylol urea (to insolubilize the casein) v 1.75

The paper base sheet or web bearing the pigmented hydrophilic adhesive coat of the foregoing Formula 2 thereon may then be suitably dried as, for example, at a temperature of from 120 F. to 140 F. for a period of from 10 to minutes, and calendered, although calendering is not essential.

Step 5.--The thus coated and calendered paper base sheet may then be washed, on the side having the pigmented hydrophilic adhesive coat of Step 4, with an aqueous solution containing zinc acetate or formate and one of the mentioned multivalent metal salts similar to that of the following formula:

The sheet is then again dried.

Step 6.-The dried coated and treated sheet may then be rubbed against rotating brushes. While not essential to or indispensable in the practice of the present invention, this operation of rubbing or brushing the face of the printing surface of the new paper base planographic printing plates increases the image-reproducing life of the plates and improves the quality of the images reproduced from the treated plates, especially when casein is the hydrophilic adhesive used. Moreover, this operation of rubbing or brushing the face of the plate tends to remove any excess of the metal salts, which may crystallize on the surface of the plate. The improvement in paper base planographic printing plates which resides in rubbing or brushing the printing surface of such plates forms the subject matter of Us. Patents 2,534,588 and No. 2,707,359.

The butanol employed in Formula 3 of Step 5 preceding, while not essential in the practice of the invention, facilitates and promotes penetration of the water-soluble salt solution into the capillary spaces formed by the pores or interstices among the finely divided'mineral particles in the coating. In place of butanol, equivalent volatile water-miscible solvents including methanol, ethanol, propanol and the like may be used.

The formaldehyde included in Formula 3 of Step 5 preceding is not essential in practice of the invention, and is used to hasten the attainment of water-insolubility of the casein in the pigmented hydrophilic coating which otherwise might take somewhat longer to develop.

The clay-casein ratio in the clay-casein hydrophilic coatingof foregoing Formula 2 may be varied widely from about 12 to 75 parts of casein per parts of clay, and the weight of the clay-casein coating may be varied from 6 to 30 pounds per ream of paper.

In place of the clay-casein hydrophilic coating referred to in Formula 2 for use in practicing the present invention, other suitable pigmented hydrophilic adhesive coatings which may be employed include those containing the following hydrophilic adhesive base materials: glue, gelatine, soybean protein, zein, modified starch, polyvinyl alcohol, carboxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, gum arabic or any other equivalent hydrophilic adhesive base material which may be pigmented and rendered water-insoluble while still remaining hydrophilic.

Moreover, the clay or pigment component of the pigmented hydrophilic adhesive or clay-casein referred to in foregoing Formula 1 may be replaced in whole or in part by other suitable finely divided mineral pigments including barium sulfate, calcined clay, talc, titanium dioxide, zinc oxide and other equivalent finely divided mineral pigments.

In place of the zinc acetate of Formula 3 of Step 5 preceding, zinc formate can be used. In place of the zirconium acetate any of the before mentioned multivalent metal salts may be substituted. In general either of the types of salt may be used in solutions ranging from about 2 percent concentration to about 25 percent concentration or to the limit of the solubility range in cases where it is lower than 25 percent. The usable ratio of multivalent salt to zinc salt will in general vary between about 1 to 9 and 9 to 1, depending upon the properties desired in specific instances. Usually the preferred range will be 'between about 1 to 4 and 4 to l.

Formula 4: Parts by weight Pigment (titanium dioxide and clay) 100 Hydrophilic adhesive (sodium carboxymethyl cellulose) 11.1 Melamine-formaldehyde partially condensed resin 5.5 Tergitol 4 1.0 Tributyl phosphate 0.5 Water 170 Phosphoric acid to adjust pH value to 5.5.

The so-coated sheet or web may then be dried and calendered, although calendering is not essential.

Step 5.-The thus coated and calendered paper sheet may then be washed, on the side having the pigmented hydrophilic adhesive coating of Step 4, with an aqueous solution containing the acetate or formate of zinc and a multivalent metal salt similar to that of the following formula:

Formula Parts by weight Aluminum formate 9 Zinc formate 3 Butanol 1.5 Tergitol 4 1.5 Water 85 The sheet is then again dried, and calendered if desired.

In Example 2, the phosphoric acid used in Formula 4 is an acid catalyst to promote further condensation of the melamine-formaldehyde reaction product whereby the hydrophilic adhesive is rendered insoluble. The Tergitol 4 in Formulas 4 and 5 is a wetting agent (sold by Carbide and Carbon Chemicals Co.), to ensure that the under surface is thoroughly wetted by the composition containing the Tergitol 4 and is a sodium alkyl sulfate said to have the formula The tributyl phosphate of Formula 4 and the 'butanol of Formula 5 are antifoam agents.

The following table shows various examples of combinations of multivalent metal salts and zinc salts in solution which may be used with satisfactory results in preceding Formulas 3 and 5, in place of the particular salt combinations shown in those formulas.

Table Percent by weight Zirconium acetate (zirconyl acetate) 20 and zinc acetate 5 Zirconium acetate 2 and zinc acetate 18 Zirconium acetate 15 and zinc formate 5 Aluminum formate 12 and zinc formate 3 Aluminum formate 5 and zinc acetate Aluminum acetate 8 and zinc acetate 8 Aluminum sulfate 18 and zinc acetate 2 Aluminum sulfate 5 and zinc formate 10 Aluminum chloride 10 and zinc acetate 10 8 Ferric formate l3 and zinc formate 2 Cerium chloride (CeCl 15 and zinc acetate 5 In place of the simple salts shown in the preceding table, double salts may be substituted, such as EXAMPLE 3 Steps 1, 2 and 3 may be identical with those of Example 1.

Step 4.--The coated paper web of Step 3 may be coated on top of the barrier coat with about 12 pounds dry weight per ream of the mixture having the composition shown in Formula 6 following:

Formula 6: Parts by weight Pigment (clay) Hydrophilic adhesive (hydroxyethyl cellulose) 30 Melamine-formaldehyde, partially condensed (Paper Products resin #4008) 10 Zirconium acetate 20 Zinc acetate 20 Tergitol 4 (wetting agent) 1 Butanol (anti-foam agent) 1 Water 200 Monochloroacetic acid to make pH value 5.5.

1 Paper Products resin N0. 4008 is sold by American Cyanamid C0.

The coated web may be dried and heated briefly at 200 C. to ensure insolubilization of the adhesive by the melamine-formaldehyde resin. The web then may be calendered and used as a planographic printing plate.

EXAMPLE 4 Steps 1, 2 and 3 may be the same as corresponding Steps 1, 2 and 3 of Example 1.

Step 4.--The coated web of Step 3 may be coated on top of the barrier coat with about 12 pounds dry weight per ream of the mixture having the composition shown in Formula 7 following:

Formula 7: Parts by weight Clay 200 Hydroxyethylated starch (Penford Gum 280) 45 Melamine-formaldehyde product (Paper Products No. 4008) 10 Aluminum sulfate 20 Zinc acetate 20 Tergitol 4 l Butanol 1 Water 200 The coated web may be dried and heated briefly to about 200 C. to ensure insolubilization of the adhesive by the melamine-formaldehyde resin. The web may then be calendered and used as a planographic printing plate.

It is to be noted that in each of the preceding examples Step 3 has provided a web having thereon a a barrier coat of strongly oleophilic character. To ensure complete coverage of the barrier coating, a relatively heavy coating (10-14 pounds per ream) of hydrophilic coating is applied in Step 4 of each of the examples. The hydrophilic coating may be applied as a single layer or in two or more layers as desired. Moreover, if desired, the successive layers of hydrophilic coating may be of dilferent composition. For instance in Step 4 of Examples 2, 3 and 4 there may be applied a first layer of from 5 to 7 pounds dry weight of a composition like that in Step 4 of Example 1. After that has dried there may then be applied enough of the composition given in Step 4 of Example 2, 3 or 4 to make a total weight over the barrier coat of from 10 to 14 pounds.

I claim:

1. An unimaged planographic printing plate comprising a paper base and a coating thereon hav'ng a planographic printing surface, said coating comprising an insoluble hydrophilic adhesive and a finely divided inert mineral pigment and having therefn capillary interstices and pores containing the evaporation residue of a solution of a divalent metal salt and a multivalent metal salt, the Weight ratio of said salts to each other being within the range from 1 to 9 to 9 to 1.

2. An unimaged planographic printng plate comprising a paper base and a coating thereon having a planographic printing surface, said coating comprising insoluble hydrophilic adhesive material and finely div'ded inert mineral pigment and having therein capillary interstices and pores containing the evaporation residue of an aqueous solution of a zinc salt of the group consist'ng of zinc acetate and zinc formate and amultivalent metal salt of the group consisting of aluminum acetate, aluminum chloride, aluminum formate, aluminum sulfate, cerium chloride, ferric formate, and zirconium acetate, said salts being present in substantial excess of the quantity capable of reacting with the adhesive material, the weight ratio of said zinc salt to said multivalent metal salt 1ying between 1 to 9 and 9 to 1.

3. An unimaged planographic printing plate comprising a paper base and a coating thereon having a planographic printing surface, said coating comprising insolubilized casein adhesive and finely divided inert mineral pigment and having therein capillary interstices and pores containing a water-soluble mixture of zirconium acetate and a zinc salt of the group consisting of zinc acetate and zinc formate, the quantity by weight of zinc salt amounting to a least one-ninth of the quantity by weight of zirconium acetate and to not more than 9 times the quantity by Weight of zirconium acetate.

4. An unimaged planographic printing plate comprising a paper base and a coating thereon having a planographic printing surface, said coating compris'ng insolubilized carboxymethyl cellulose and finely divided inert mineral pigment and having therein capillary interstices and pores containing a water soluble m'xture of aluminum sulfate and a zinc salt of the group consisting of zinc acetate an zinc formate, the quantity by weight of zinc salt amounting to from one-ninth to nine parts per part by weight of aluminum sulfate present.

5. An unimaged planographic printing plate comprising a paper base and a coating thereon having a planographic printing surface, said coating comprising insolubilized carboxymethyl cellulose and finely divided inert mineral pigment and having therein capillary interstices and pores containing a water soluble mixture of aluminum formate and zinc formate in the weight ratio of about 3 to 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,534,650 Worthen Dec. 19, 1950 2,635,537 Worthen Apr. 21, 1953 FOREIGN PATENTS 475,219 Canada July 10, 1951 

1. AN UNIMAGED PLANOGRAPHIC PRINTING PLATE COMPRISING A PAPER BASE AND A COATING THEREON HAVING A PLANOGRAPHIC PRINTING SURFACE, SAID COATING COMPRISING AN INSOLUBLE HYDROPHILIC ADHESIVE AND A FINELY DIVIDED INERT MINERAL PIGMENT AND HAVING THERE''N CAPILLARY INTERSTICES AND PORES CONTAINING THE EVAPORATION RESIDUE OF A SOLUTION OF A DIVALENT METAL SALT AND A MULTIVALENT METAL SALT, THE WEIGHT RATIO OF SAID SALTS TO EACH OTHER BEING WITHIN THE RANGE FROM 1 TO 9 TO 9 TO
 1. 